RE: [RFC PATCH v4 2/6] perf stat: Fork and launch perf record when perf stat needs to get retire latency value for a metric.

[Wang, Weilin]

> -----Original Message-----
> From: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> Sent: Tuesday, March 12, 2024 5:56 PM
> To: Wang, Weilin <weilin.wang@xxxxxxxxx>
> Cc: Namhyung Kim <namhyung@xxxxxxxxxx>; Ian Rogers
> <irogers@xxxxxxxxxx>; Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>; Peter
> Zijlstra <peterz@xxxxxxxxxxxxx>; Ingo Molnar <mingo@xxxxxxxxxx>;
> Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>; Jiri Olsa
> <jolsa@xxxxxxxxxx>; Hunter, Adrian <adrian.hunter@xxxxxxxxx>; Kan Liang
> <kan.liang@xxxxxxxxxxxxxxx>; linux-perf-users@xxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; Taylor, Perry <perry.taylor@xxxxxxxxx>; Alt, Samantha
> <samantha.alt@xxxxxxxxx>; Biggers, Caleb <caleb.biggers@xxxxxxxxx>
> Subject: Re: [RFC PATCH v4 2/6] perf stat: Fork and launch perf record when
> perf stat needs to get retire latency value for a metric.
> 
> "Wang, Weilin" <weilin.wang@xxxxxxxxx> writes:
> 
> >> -----Original Message-----
> >> From: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> >> Sent: Tuesday, March 12, 2024 5:03 PM
> >> To: Wang, Weilin <weilin.wang@xxxxxxxxx>
> >> Cc: Namhyung Kim <namhyung@xxxxxxxxxx>; Ian Rogers
> >> <irogers@xxxxxxxxxx>; Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>;
> Peter
> >> Zijlstra <peterz@xxxxxxxxxxxxx>; Ingo Molnar <mingo@xxxxxxxxxx>;
> >> Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>; Jiri Olsa
> >> <jolsa@xxxxxxxxxx>; Hunter, Adrian <adrian.hunter@xxxxxxxxx>; Kan Liang
> >> <kan.liang@xxxxxxxxxxxxxxx>; linux-perf-users@xxxxxxxxxxxxxxx; linux-
> >> kernel@xxxxxxxxxxxxxxx; Taylor, Perry <perry.taylor@xxxxxxxxx>; Alt,
> Samantha
> >> <samantha.alt@xxxxxxxxx>; Biggers, Caleb <caleb.biggers@xxxxxxxxx>
> >> Subject: Re: [RFC PATCH v4 2/6] perf stat: Fork and launch perf record
> when
> >> perf stat needs to get retire latency value for a metric.
> >>
> >> weilin.wang@xxxxxxxxx writes:
> >>
> >> > From: Weilin Wang <weilin.wang@xxxxxxxxx>
> >> >
> >> > When retire_latency value is used in a metric formula, perf stat would fork
> a
> >> > perf record process with "-e" and "-W" options. Perf record will collect
> >> > required retire_latency values in parallel while perf stat is collecting
> >> > counting values.
> >>
> >> How does that work when the workload is specified on the command line?
> >> The workload would run twice? That is very inefficient and may not
> >> work if it's a large workload.
> >>
> >> The perf tool infrastructure is imho not up to the task of such
> >> parallel collection.
> >>
> >> Also it won't work for very long collections because you will get a
> >> very large perf.data. Better to use a pipeline.
> >>
> >> I think it would be better if you made it a separate operation that can
> >> generate a file that is then consumed by perf stat. This is also more efficient
> >> because often the calibration is only needed once. And it's all under
> >> user control so no nasty surprises.
> >>
> >
> > Workload runs only once with perf stat. Perf record is forked by perf stat and
> run
> > in parallel with perf stat. Perf stat will send perf record a signal to terminate
> after
> > perf stat stops collecting count value.
> 
> I don't understand how the perf record filters on the workload created by
> the perf stat. At a minimum you would need -p to connect to the pid
> of the parent, but IIRC -p doesnt follow children, so if it forked
> it wouldn't work.
> 
> I think your approach may only work with -a, but perhaps I'm missing
> something (-a is often not usable due to restrictions)
> 
> Also if perf stat runs in interval mode and you only get the data
> at the end how would that work?
> 
> iirc i wrestled with all these questions for toplev (which has a
> similar feature) and in the end i concluded doing it automatically
> has far too many problems.
> 

Yes, you are completely right that there are limitation that we can only support -a, -C 
and not support on -I now. I'm wondering if we could support "-I" in next step by 
processing sampled data on the go.

Thanks,
Weilin

> -Andi